A microRNA (or miRNA) is a small molecule RNA composed of 18 to 25 bases, and binds to a target mRNA to inhibit the translation of the target mRNA into a protein. About 1100 types of human miRNAs have been registered in databases (e.g. miRBase (http://www.miRbase.org/)) so far, and it has been clarified that abnormalities in expression or functions of these miRNA are involved in various types of diseases (see Non-Patent Document 1). Particularly relating to cancers, many miRNAs whose expression is increased or suppressed in cancer tissues or cells have been identified by comprehensive expression analysis using miRNA microarrays (Patent Document 1).
As miRNAs whose expression increases in cancers, miR-21, miR-155, miR-17-5p, miR-19, and the like have been known. Among these, miR-19 is necessary and sufficient for cell tumorigenesis, and one of its targets is proved to be the cancer suppressing gene PTEN. Such a miRNA that has a property like an oncogene against a tumor suppressor gene is called “OncomiR”.
Further, in the cases of many miRNAs, expression thereof decreases in cancers. As such miRNAs, let-7, miR-15a, miR-34a, miR-143, miR-145 and the like have been reported. Regarding let-7 among these, it has been proved that in lung cancer the decrease of expression of let-7 and the prognosis correlate each other, and it has been known that the targets thereof include the oncogene Ras. A miRNA that functions like a tumor suppressor gene against an oncogene is called “tumor suppressor miR”.
On the other hand, attempts have been made to inhibit miRNA involved in diseases, such as the OncomiR, as the target molecule. For example, the following has been proposed: by introducing an antisense oligonucleotide having a sequence complementary to a target miRNA into cells, a double strand is formed with a target miRNA in each cell so that the function of the target miRNA is inhibited (Patent Document 2).